In the course of assessing and producing hydrocarbon bearing formations or reservoirs, it is important to acquire knowledge of formation and formation fluid properties which influence the productivity and yield from the drilled formation. In many cases such knowledge is acquired by methods generally referred to as “logging”.
Logging operations involve the measurement of a formation parameter or a formation fluid parameter as a function of location, or more specifically depth (as measured along the length of the well). Formation logging has evolved to encompass many different types of measurements including measurements based on sonic, electro-magnetic or resistivity, and nuclear measurements such as nuclear magnetic resonance or neutron capture effects.
Nuclear magnetic resonance (NMR) methods are well established in the laboratory to measure fluid flow in pipes, rocks, and biological systems. Typically, flow is measured by encoding molecular displacements in the phase of nuclear spins in the (flowing) fluid, during an evolution interval Δ. An encoding and decoding of the spin's position before and after interval Δ is affected by means of static or pulsed field gradients. The NMR signal is measured for either a range of phase encoding times or a range of gradient strengths, or both, and then analyzed. For small static or pulsed field gradient strengths or encoding times, the shift of the phase of the NMR signal is proportional to the velocity and the time which has elapsed between the encoding and the decoding steps. The complete probability distribution of molecular displacements during the evolution period Δ can be obtained for example from pulsed field gradient-NMR (PFG-NMR) by extending the measurements to larger pulsed field gradient strengths, then Fourier transforming the data. The PFG-NMR type of experiment has been called NMR-scattering in the published literature, and the extraction of probability distributions from such experiments is commonly referred to as a measurement of the propagator in the published literature.
NMR measurements are also commonly used in the borehole to probe the NMR decay behavior of the stationary fluid in the reservoir rock. In these techniques, magnetic fields are established in the formation using suitably arranged permanent magnets. These magnetic fields induce nuclear magnetization, which is flipped and otherwise manipulated using on-resonance radio frequency (RF) pulses. NMR echoes are observed, and their dependence (of their magnitude) on pulse parameters and on time is used to extract information about the formation and the fluids in it.
In particular, NMR has been established in the oilfield industry to obtain information on bound water, free water, permeability, oil viscosity, gas-to-oil ratio, oil saturation and water saturations. All of this information can be derived from measurements of spin-spin relaxation time often referred to as T2, spin-lattice relaxation time (T1), and self-diffusion coefficient (D) of the hydrogen containing formation fluids.
Some of the known NMR tools can be controlled so as to acquire data from different radial depth layers in the formation. These measurements at multiple depths of investigation (DOIs) are usually taken to provide a good signal-to-noise ratio. In some instances, as for example, described in the co-owned U.S. Pat. No. 6,703,832 issued to Heaton and Freedman, differences of the NMR measurements taken at different DOIs are analyzed to determine conditions within the formation.
On the other hand, fluids are routinely sampled in the borehole using various known fluid sampling tools, such as Schlumberger's MDT™. The MDT tool when set up for sampling includes at least one fluid sample bottle, a pump to extract the fluid from the formation, and a contact pad with a conduit to engage the wall of the borehole. When the device is positioned proximate a region of interest, the pad is pressed against the borehole wall, making a tight seal. Fluid in the formation is induced to flow, by pumping fluid out of the formation through the hole in the pad.
Fluid in the formation can also be moved with the help of known dual packer systems. The dual packer system seals off a section of the borehole (on the order of feet) with inflatable packer elements. When a dual packer is used to pump fluid from the formation, fluid will be extracted from the borehole section which is sealed off.
Samples of the fluid are either analyzed in-situ within the body of the tool or placed into a sample bottle for later analysis. The module or dual-packer section is then moved to the next region of interest (station). Information regarding the movement of fluid in the formation during the pumping process can provide valuable information related to formation and fluid sample properties.
Fluid flow towards the borehole is also routinely produced during pressure testing, essentially in the same manner as described for the MDT tool described above. Accordingly, useful information may be similarly obtained during this process.
It is further well established to mount the measurement tools for a logging operation on either dedicated conveyance means such as wireline cables or coiled tubing (CT) or on the drilling string. The latter case is known in the industry as measurement-while-drilling (MWD) or logging-while-drilling (LWD). In MWD and LWD operations the parameter of interest is measured by instruments typically mounted close behind the bit or the bottom-hole assembly (BHA). Both, logging in general and LWD are techniques known for decades and hence require no further introduction.
Combinations of a flow generating tool such as the MDT with tools for performing NMR measurement are described in a number of published documents including for example in co-owned U.S. Pat. No. 7,180,288 to Scheven. Another detailed description of possible NMR-based methods for the purpose of monitoring flow and formation parameters can be found in co-owned U.S. Pat. No. 6,642,715 to Speier et al. and U.S. Pat. No. 6,856,132 to Appel et al. A tool which combines a fluid injection/withdrawal tool with a resistivity imaging tool is described for example in U.S. Pat. No. 5,335,542 to Ramakrishnan et al.
In an paper prepared for presentation at the SPWLA 1st Annual Middle East Regional Symposium, Apr. 15-19, 2007 Gilles Cassou, Xavier Poirier-Coutansais, and Raghu Ramamoorthy, demonstrate that the combination of advanced-NMR fluid typing techniques with a dual-packer fluid pumping module can greatly improve saturation estimation in carbonates. The ability to perform 3D-NMR stations immediately before and after pump-outs yields both the water and oil saturation Sw and Sxo, independently of lithology, resistivity, and salinity, in a complex carbonate environment.
In view of the known art, it is seen as one object of the invention to improve and enhance the effectiveness of NMR based tools or other wellbore tools for the purpose of characterizing the formation and its fluid content using measuring apparatus with a volume of investigation overlapping or co-located with the volume in which the induced flow occurs.